Diversity reception



P0 o0 A. nu e, 2

July 22, 1952 1 R. KAHN DIVERSITY RECEPTION Filed April 2s, 1949.

ATTO RN EY Patented July 22, 1952 DIVERSITY RECEPTION Leonard R. Kahn, New York, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 28, 1949, Serial No. 90,241

, Claims.

This invention relates to diversity receiving systems, and more particularly to systems for obtaining diversity reception of frequency shifted transmitted signals. f

An object of this invention is to devise a diversity receiving system in which the changeover in the connection of the output circuit to one receiver or another may be automatically effected in response to very small differences in receiver signal strengths, if desired.

Another object is to provide a diversity receiving system which is rather simple in design and therefore relatively inexpensive.

A further object is to provide a diversity receivingl system in which vdiversification is accomplished in very short periods of time, without the use of trigger circuits, which may be unreliable in some cases.

The foregoing and other objects of the invention will be best understood from the following description of an exemplication thereof, reference beingv had to the accompanying drawing, wherein the single figure is a block diagramof a receiving system according to this invention.

The objects of the invention are accomplished, briefly, in the following manner:

TheV ,output of one receiver of a pair of receivers in space diversity is mixed or heterodyned with the output of a fixed frequency source, after which this filtered resultant output and the output of the other receiver are both fed to a mixer, inwhich they are both heterodyned to a higher frequency by means of another fixed frequency source. In the output ofthe latter mixer there are two fundamental or rest frequencies of slightly different values, representingl the outputs of the two receivers, each rest frequency varying in frequency in' accordance with the keyed frequency shifted signals being received. A driven, lockedin or slave oscillator, having a center or rest frequency'between the two said fundamentalfrequencies, is connected to the output of the latter mixer to be entrained thereby. This oscillator is .entrained bythe stronger ofthe twoA signals fed thereto and therefore operates or oscillates at the frequency of the stronger signallwhch in turn corresponds to the strongerorv more favorable output signal of the two diversity receivers.. The output of the slave oscillator is passed through a limiter and is then appliedto'two discriminators,

(Cl. Z50-20) one of which iscenteredat one-of the said fundamental frequencies and theother ofwl'iic'h .is centered at the; other said'ffundar'nental ,'frequency. These. discriminators function yalternatively, '.in ,accordance withthe frquencyof the slave voscil- 2 lators output, to convert the frequency shifted signals to amplitude-varying output signals, which are supplied to a common output circuit. The discriminators supply ,cross-connected AVC voltages to the receiver to aid in the switching or changeover from one diversity receiver to the other. I

' Now referring to the drawing, which illustrates schematically' a system according to this invention, receiving antennas I and 2 are in space or phase diversity with respect to a distant transmitter, so thatin general the strengths of the transmitted signals picked up by these two antennas are unequal; at any instant, one of these antennas is relatively more favorably located with respect to the transmitter than is the other. The signal picked up by antenna l is fed to a receiver 3 for channel A, while the signal picked up by antenna 2 is fed to a receiver 4 for channel B. In each of these receivers, the received signal is amplified and is heterodyned down to a suitable intermediate frequency, such as 50 kilocycles for example, so that in the output of each of the receivers 3 and 4 there appears a signal whose frequency is 50 kilocycles plus or minus the frequency shift of the transmitted signal, this frequency shift being keyed at the transmitter to represent intelligence.

The output of receiver 4 is mixed with the output of an oscillator 5, having a frequency of 10 kilocycles for example, in a mixer and filter unit 6. From such mixing there results, as is well known,.frequencies of kilocycles and 40 kilocycles, both frequencies being plus or minus the frequency shift. The filter in unit 6 passes the upper sideband of 60 kilocycles iFS to another mixer and lter unit 1. The output of receiver 3 `is supplied directly to unit 1, so that there are supplied, to such mixer, frequencies of 60 kilocycles l-FS and 50 kilocycles FS. In mixer 1, the signal voltages are mixed with the output of an oscillator 8, which may have a frequency of 1 megacycle or 1000 kilocycles, for example, to produce frequencies of 1060 kilocycles iFS and 1050 kilocycles iFS,.the other mixing or modulation products being filtered out bythe filter in unit 1. The 1060-kilocycle signal is representative of, and proportional in strength` to, the output of receiver 4', while the 1050-kilocycle signal is representative of, and proportional in strength to, theoutput of receiver 3. At any particular instant, the version of the received -signal in receiver 3 may be stronger and'more favorable, while at some other instant the version in receiver 4 may be stronger and more favorable, and

'of the controlling'voltages fed thereto which has the greatest amplitude, provided, of course, that' such frequency is within the'lock-in range of; the oscillator. The oscillatorS has a center or rest frequency of 1055 kilocycle's, andthe out-fl put of unit 1 is fed as a controlling Voltage to such oscillator. The oscillator 9 willY be entrained by, or will fall into synchronization with,

the frequency of the stronger controllingvoltage Z ,Y or signal fedthereto from unit '1, and since these di'iferent frequencies of theeontrolling voltage are derived from corresponding separate divers i' t 5` 1A receivers, diversity switchingV action is V0b-` tainduu .'Ilhe outputof oscillator 9 will be either 1050Vlilocycles HfFSor 1060 kilocycles i' FS, according to whether receiver VA or receiver B recives the stronger signal.

{I'he keyed frequency shifted output of 'oscile lat r 1 9' is Vfe'dy'through a limiter l0 to :remove amplitudevariations andris then fed'to a dis` criminator ll for channel A and to a discriminate'r |72 for channel B. 'he locked-in oscillator 9 has limiting characteristics, as described iii Ythe BeersV patent referred to, but an extra liiniter I is provided in orde'rto simplify somewhat the design of said oscillator. Discrimiiiatos Il and I2 are both of the so-called ratio detector type described in an article by S. W.

Seeley, appearing in Broadcast News, No. 42,

January; 194s, pages #ls-47; discriminator 1| is which are -fd to a suitable Aoutput device. When oscillator 9 is so entrained, discriminatoriB remains Acoifipletely or substantially completely outof the picture, since it is centeredA at 1060 kilocyls-'and has an inappreciable output when 1050-'kiloc-yc`le signal is applied' to the input thereof. j

i When -oscillator 9 is locked-in by the 106 Okilo' cycle f- -FS signal deriv'edfrom receiver B, v[and th'e'i' re Nhas an voutput of such frequency, dis,- criniiiator B (which lis centered at r1060 kilojfunctie'ris in a similar manner to Y'convert the frequency-varying or frequency-shifted s i'gnalswat its'input to amplitudevarying signals, whichvare fed to theoutput device'. :When o scillatome 4is so entrained, discriminator A .rel njiainssubstantially completelyput' of rthe picture,v it being centered at 1050 kilocycl'esj and kllcy'cle signal is applied to its iniciut.`

Titinay be seen, from the above ,rthat thes'lave 1050-kilocycle signal being stronger when the signal in receiver A is stronger and the 1060- kilocycle signal being stronger when the signal in receiver B is stronger. Thus, diversity reception and automatic switching is provided without the use of trigger circuits, by relatively simpie equipment.

In order to facilitate the Yswitching of the locked-in oscillator 9 from the control of one Areceiver to the other when the two inputs to such oscillator are equal or approximately equal,

a cross-connected AVC arrangement is utilized.

From a suitable point in the ratio detector ll, an AVC potentialis taken off and applied to the amplifying stages in receiver d, in a conventional manner, to control the gain thereof in accordance with the average strength of the signal supplied to such detector. Such a suitable vpoint in the detector may beV found by ref` erence to Fig. 2 of the aforementioned Seeley article. Similarly, an AVCfpotential is taken oir froma suitable' point in detector l2 and applied to the amplifying stages in receiver 3 to control the gain thereof in accordance with averageV strength of the signal supplied to said detector. f

A"When discriminator A is functioning that is, when the signal from receiver A is stronger, an AVC potential is produced in said discriminator, which potential reduces the gain of receiver B by a predetermined amount, for example 3 db. Discriminator B controls receiver A in a similar manner. Let u's assume thaty receiver Ahas control of the circuit. In order for receiver B to take control of the circuit, the signal coming into receiver B must be 3 db stronger than the signal coming' into receiver A, since at this time the gain in receiver B is reduced B'Ydb by the AVC vconnection from discriminator A i Just beyond the point 'Wh'ererec'eiver B has this 3 db superiority, the 1030'-kilocycle signal yin the output o 'funitl will be slightly stronger than ythe 1050-kilocycle signal, so that the for-A nierV will take control of oscillator S and discriminator- B will start lt'oliunc'tion. Momentarily, and at this instant,"because` of the'crossconnected YAVC arrangement, the ltworeceivers A and B'will' have the'saniegain and the 1060'- kilocycle signal in the outpt'it' of unit." will then exceed the 1050-kilocyclefsi'gnal by .3 db, since the signal coming-.into receiver'B isB db 'stronger than Ythe Signal Corning into Ie'cleiverV A.l t YSince thegslavefroscillator Sloan easily differentiate between two signals in the output of unit 1 which differ in strength by 3 db, no' signal will appearjin the routput of discriminatorA and its AVC potential will be reduced to zero, increasing the'gain of receiver B by 3 db." This actioriivill'saop the Strength f the'lOGOK-kiloyole signal the output'ofunit 1 -to a level 6 db higher thanY that of the 1050-kilocycle -sinal in suchL output, causing switching or changeover toiftake place in a very small period of time.

Bysuitable' design of the discriminator circuits, the AVC potentials mayV be madeto Vhave 'a value such as to cause 'changeover to take 'place atea ve'IyV-'s'inalll diiernce in vreceivedsignial strengths, ifso desired; Y' i l f Th" vstern of this invention can be sedffor tri" diversitx'aswellas for double diversity as illustrated; In triplel diversity, the output of the thi dre'ceiver; would be'inixe'd with a 2 0-kilocyc`le wavegfi'o'm @suitablesource-andthe three fref vquenciesf 105,04 kilocy'cles. 1060 kilocycl'es, and

ioio'kilo'cycies, would' be' "fed t'oalocked-in oscillator centered atlO'GOkilocycles. The 1060:! kilocycle frequency, being .atsthencenterfre-,r quencyof thev locked-in oscillator, lwouldfhave to be attenuated somewhat. c f fl vWhat I claim'to'bemy invention is as'follows:

J1. In a Adiversity receiving system for fre'- quencyfshifted signals, a plurality of. diversity receivers,v heterodyningzmeans for converting the receiver outputs to waves having frequencies differing from each other, a slave oscillator adapted to be entrained to operate at'thefrequency of. the strongest controlling voltage `fed thereto, means for feeding saiddiiferingfrequencywaves to :said oscillator as controlling voltages therefor,

and-'a plurality of .discriminators vhaving their. inputsv coupled'to the Voutput of .said oscillator and their outputs coupled togetherto a common circuit, 4eachof said-discriminators being tuned to ,aorresponding one of said. differing', fre--r quencies. Y 1 n 2. In a diversity receiving system forfrequency shifted signals, a'plurality of diversity receivers, heterodyning means for converting thereceiver outputsto waves having frequencies differing from each other, a slave oscillator adapted to be entrained to operate at jthe.,A frequency of the strongest controlling voltage fed thereto,` means for feeding said differing frequency waves to said oscillator as controlling voltages therefor, a limiter coupled to the output`=of`saidoscillator, and a plurality of discriminators having their inputs coupled to the output ofvsaidlimiter and-their outputs coupled together to ajeommon circuit, each ofsaiddiscriminators being tuned to a correspondngone of said differing frequencies.

l3. In a, diversity receiving'system for frequency shifted signals, a pair of diversity receivers, heterodyning means for converting the output. of one receiver to a vwave having a frequency differing from that of the other receivers output, a slave oscillator adapted to be entrained to operate at the frequency of the stronger controlling voltage fed thereto, means for feeding the two differing frequency waves to said oscillator as controlling voltages therefor, and a pair of discriminators having their inputs coupled to the output of said oscillator and their outputs coupled together to a common circuit, one of said discriminators being tuned to the frequency of said Wave and the other of said discriminators being tuned to a frequency representative of the output of said other receiver.

4. In a diversity receiving system for frequency shifted signals, a plurality of diversity receivers, separate heterodyning means for converting the receiver outputs to waves having frequencies differing from each other, common heterodyning means for converting all of said differing frequency waves to other waves having frequencies which differ lfrom each other by amounts equal to the original frequency differences, a slave oscillator adapted to be entrained to operate at the frequency of the strongest controlling voltage fed thereto, means for feeding said other differing frequency waves to said oscillator as controlling voltages therefor, and a plurality of discriminators having their inputs coupled to the output of said oscillator and their outputs coupled together to a common circuit, each of said discriminators being tuned to a corresponding one of said other differing frequencies.

5. In a diversity receiving system for frequency shifted signals, a plurality of Vdiversity receivers, separate heterodyning means for converting the receiver outputs to waves having frequencies differing from eachother, common het: erodyningmeans for convertingall of said differing frequency` waves to. other waves having frequencies .which differ from veach other vby amountsequal to theoriginal frequency differences, a slave oscillator adapted to be entrained to operatel at the frequency of the vstrongestcon-` trolling voltage fed. theretmmeans forA feeding said ,other differing frequency waves .tosaid os. cillator as controlling voltages therefor, a limiter coupled to the output of said oscillator, and a plurality of discriminators having their inputs coupled to` the output of saidl lirnitenandtheir. outputs, coupled Atogether v-to' a common .ciruit,

other waves having frequencies which differ f-ro m each other fby Y an amount .equaleto' Said named Vfrequency difference, ,a slave oscillator adapted `to be entrained to, operate atthemfrequency of; the stronger controlling., voltage fed thereto, means for feeding said two other waves tosaid oscillator as controlling voltages therefor; a limiter coupled to the output of said oscillator, anda pair of discriminators having their inputs coupled to the output ofsaid limiter andtheir outputs coupled' together to a common circuit, one of, said discriminators being tuned to the frequency of oneof said two other waves. andthe other of said discriminators being-tuned tov the frequency of the other of said two other waves.

'7. In a diversity receivingA system for frequency shiftedv signals, a- -pair of diversity receivers, heterodyning means for converting the output of one receiver to a wave having a frequency differing from that of the other receivers output, a slave oscillator adapted to be entrained to operate at the frequency of the stronger controlling voltage fed thereto, means for feeding the two differing frequency waves to said oscillator as controlling voltages therefor, a pair of discriminators having their inputs coupled to the output of said oscillator and their outputs coupled together to a common circuit, one of said discriminators being tuned to the frequency of said wave and the other of said discriminators being tuned to a frequency representative of the output of said other receiver, said discriminators each developing therein a potential proportional to the average strength of the signal in its output,` connections for applying the developed potential of said one discriminatorto said other receiver to reduce the gain thereof, and connections for applying the developed potential of said other discriminator to said one receiver to reduce the gain thereof.

8. In a diversity receiving system for frequency shifted signals, a pair of diversity receivers, heterodyning means for converting the output of one receiver to a wave having a frequency differing from that of the other receivers output, common heterodyning means for converting said wave and the output of said lother receiver to two other waves having frequencies which differ from each other by an amount equal to said first-named frequency difference, a vslave oscillator adapted to be entrained to operate at the frequency .of .the i'stronger controlling .voltage fedr'thereto', meanslfonfeeding saidtwo other Y ini-'aw-potential proportional to the" average strength;Y ofV the# signalyi'n Ait'sf output, connections for `applying the developed'potential of saidlone discriminator to said other receiver to reduce' the gain thereof,V andY connections for applying the developed potential ofsaidY other discriim'inator` tosaid' one receiver to reduce the gain` thereof.

l9. Inaldiv'ersityrrecevin'g systemfor frequency shifted signals, a pair of diversity receivers,- heterodyning meansforl converting the output ofi one receiverto awave having a' frequency diffcmxnon-'hetrodyning means for converting said` Wave'and theoutput of'said;y other receiver to twoother waves having frequencies which diner from earch other by*anamountV equal to saidlflr'stnamed lfrequencydiier'ence, a slave oscillator adapted to be entrained tooperate at the fre'- quency of the stronger controlling voltage fed` thereto, means for feeding said tuloV vother Waves to'said oscillator asi controlling .voltages therefor, ailmiter coupled tothe output of 'said oseillator, a pair-fof discriminators having their inputs coupled' t0 the' output ofA said limiter and' their outputs coupled together toa common circuit,` one 'of saiddiscriminators being tuned tothe frequency ofthat'on'e of said two kother waves representing 'the' output of said one V,receiver and.

the other,v of vsaid,di'serifriin'at'ors being tuned-:5to

thefrequency of'tha-t one of saidztwofotheri-.Waves representing the output. ofsaid other receiver, said discriminators each developingl therein a potential` proportional .tothe average strength of the'signal in its output; connections for applying the developed potentialof said one discriminator to said (other receiver 13oreduce the gai-n thereof, and connectionsfor applying the developedA po-V tential ofJsaidother discrin'iina'tor4 to said :onereoeiveritoireducethe-gain thereof. v i

10. In a diversity..receivingl system, a plurality of diversity receivers-@hetero dyningv means'mior converting the receiver outputs to wavesrhaving lfrequencies differing from each V'othenla vslave oscillator coupled torv s'aidfsh'eterodyning- Inl-lansy thereby to feed said diierinfg'trequency wavessto' saidA oscillator vas v'controlling voltages therefor, said oscillator being adapted' to be 'entra-ined to operate at the frequency of the strongest con# trolling Voltageffed to' Seidl/oscillator, and dis,- crirninating means coupled-to the output olf/said slaveoscillator-f"` I i fil 1 'f n f'- -rLEQNARD R. KAHN'.- 'j

fimrimenees CITED v v Y,The following `references ,arer olie'cord in 'the file of this patent: v UNITEDeSrBATEsPA'rENTsf,

A1\h1'nr1lo"erv v Name'v -f 'l Datelv 1,395,111 j 'suyda'm- L 1Jan; 24, 193e 1,968,750 Edwards July- 3`f1, 13934 '2,004,107 Goldsmith AJune 11, 1935 2,069,813V y rlBiel/e'rageA F'ebi $5,V v1937 Y 2,073,409 shanki' Mar. 9;1-937 '2,255,668 Kocl' Sept. 9, 1941 2,356,201 Beers Aug. 22, 1944 2,440,653 Corrington AprrZ'l, v194:8

21,507,730 May `156,'1950 

